- extend norm functions

This is required when specifying norm in Petsc vector interface.

This a lightweight pack-unpack class holding two vectors.

This helps with generalization when using essentially same concept of addition.

This simplifies matrix construction in many places because in many cases we need just a bilinear form.

- Removed the update code for calculation of _max_diag_coeff from
  + LisMatrix::setValue()
  + LisMatrix::addValue().
- Removed the _max_diag_coeff attribute.
- Implemented the functionality in LisMatrix::getMaxDiagCoeff().

- Reformatted code
- Made docu more meaningful
- Changed name of first parameter of constructor
- Moved implementation of setValue() and getValue() from header into cpp-file